Motor-driven centrifugal pump for the primary circuit of small or medium-sized modular nuclear reactors

ABSTRACT

A motor-driven centrifugal pump for circulating a coolant in a primary circuit of a nuclear reactor comprises a sealed motor unit, a hydraulic part and a shaft which is immersed in the coolant, turned by the sealed motor unit and pumping the coolant by means of an impeller of the hydraulic part secured to the shaft; the motor unit comprising a dry stator and an immersed rotor mounted securely on the shaft. The motor-driven pump also comprises an immersed flywheel mounted securely on the shaft between the rotor of the motor unit and the impeller of the hydraulic part, allowing a minimum slowing-down time after the electrical power supply is cut; the flywheel comprising a vaned wheel generating a circulation of the coolant to cool the motor unit.

The present invention relates to the field of primary motor-driven pumpunits which provide the cooling of the nuclear reactor and circulate acoolant through the primary circuit, and relates more particularly to adesign of motor-driven primary pump that is suited to small andmedium-sized modular reactors.

Unlike the more powerful nuclear reactors capable of distributing anelectrical power in excess of 1000 MW, the small and medium modularreactor sector, commonly referred to as SMRs which stand for SmallModular Reactors, is undergoing substantial expansion. Many designs ofSMR are in progress, generally in the 50 to 700 MW range (<300 MW forsmall reactors; <700 MW for medium reactors). Being less expensive andmore flexible than traditional installations, SMRs provide the answer tothe increasing need for electrical energy, particularly in thedeveloping countries.

The modular construction of the SMRs allows production capacity to beincreased incrementally by using the number of modules that suits theneed. Being more compact, they can be partially prefabricated prior totransport and assembled on the final end-user site. Designed as a plugand play solution, SMR installations can be built more quickly, offeringgreater flexibility in terms of financing, in terms of the scope ofinstallation work to be carried out on site, in terms of the size andthe end-use.

There are a number of reactor types envisioned for SMRs particularlypressurized water reactors (PWRs), high temperature reactors (HTRs) oreven molten salt reactors (MSRs). In PWRs, the ordinary water thatconstitutes the coolant is kept liquid under high pressure, of the orderof 150 bar. In the primary circuit, the water collects the heat producedby the nuclear fuel and transmits it to the fluid of the secondarycircuit using steam generators. In the integrated PWRs envisioned forthe SMRs, one or more steam generator(s) is/are situated in the sameenclosures as the reactor. The heat from the reactor is transmitted bythe secondary circuit to a steam turbine which drives an alternatortasked with producing electricity.

One or more primary pumps circulate the water through the primarycircuit, between the reactor core and the steam generators. The primarycoolant pump is a component essential to the operation and safety of aPWR facility. The design of SMRs entails a rethink of the architectureof the primary pump because the architecture of an integrated PWR whichhas the reactor, the primary circuit and the steam generators in onemodule imposes new requirements, both geometric and functional, and interms of durability, or safety requirements.

The ambitious specification sheet for a reactor that is simple, compact,inexpensive and meets the most stringent safety requirements, islogically passed on to its constituent parts. Existing primary pumps areillustrated to this new requirement. By way of nonlimiting example ofthe present invention, a motor-driven primary pump suited to an SMR has,by way of functional requirements, a durability of 60 years, amotor-driven pump inlet fluid temperature of between 300 and 350° C., anoperating pressure of 140 to 160 bar, a coolant density of between 600and 700 kg/m³, and a raft of geometric requirements (e.g. the ability tomount vertically or horizontally, the overall size envelope),installation requirements (e.g. the fact that it must not be installedby welding, that it must be possible to disconnect it completely fromoutside the reactor, without components remaining inside the enclosure),and external requirements (e.g. seismic requirements).

Thus, it is desirable to have a motor-driven primary pump that meetsthese new and ambitious requirements. The suitable motor-driven primarypump will need to be of simple design, robust, economical, compatiblewith mass production, and meet the most stringent functional andregulatory requirements.

To this end, the subject of the invention is a motor-driven centrifugalpump for circulating a coolant in a primary circuit of a nuclearreactor. The motor-driven pump comprises a sealed motor unit, ahydraulic part and a shaft which is immersed in the coolant, turned bythe sealed motor unit and pumping the coolant by means of an impeller ofthe hydraulic part secured to the shaft. The motor unit comprises a drystator and an immersed rotor mounted securely on the shaft. Themotor-driven pump also comprises a flywheel, which is immersed in thecoolant, mounted securely on the shaft between the rotor of the motorunit and the impeller of the hydraulic part, making it possible toensure a minimum slowing-down time when the motor unit ceases to turnthe shaft. The flywheel comprises a vaned wheel generating a circulationof the coolant, making it possible to cool the motor unit.

Advantageously, the motor-driven pump comprises an external coolingcircuit connected to the motor unit, through which the coolant driven bythe vaned wheel circulates. The coolant is reintroduced after cooling bysaid circuit into the motor unit.

Advantageously, the external cooling circuit comprises at least one coilwound on a body of the motor-driven pump; the coolant circulatingthrough the at least one coil being cooled by means of a secondarycoolant circulating around the body in a shell.

Advantageously, the vaned wheel is made up of channels or grooves formedon a surface of the flywheel that faces toward the rotor.

Advantageously, the motor-driven pump comprises at least three radialbearings guiding the rotation of the shaft. The rotor is positionedbetween a first radial bearing and a second radial bearing. The flywheelis positioned between the second radial bearing and a third radialbearing. The circulation of coolant generated by the vaned wheel alsoallows the third radial bearing to be lubricated.

Advantageously, the first radial bearing and the second radial bearingare hydrodynamic bearings of the plain bearing, pad bearing and/orthree-lobe bearing type, and the third radial bearing is a hydrostaticbearing.

Advantageously, the motor-driven pump also comprises an axial bearingpositioned near the first radial bearing and able to block the axialmovement of the shaft.

Advantageously, the axial bearing is a tilting-pad bearing.

Advantageously, the motor-driven pump comprises a thermal barrierbetween the motor unit and the hydraulic part, in order to thermallyisolate the motor unit from the primary circuit. The shaft passesthrough the thermal barrier between the flywheel and the impeller.

Advantageously, the flywheel is made of Inconel 625.

Advantageously, the motor-driven pump comprises an anti-backspin deviceable to mechanically prevent the rotation of the shaft in apredetermined direction.

The invention will be better understood and further advantages willbecome apparent from reading the detailed description of someembodiments given by way of example in the following figures.

FIGS. 1a and 1b illustrate a motor-driven primary pump according to theinvention and how it is integrated in a vertical configuration into anSMR,

FIGS. 2a, 2b and 2c depict the motor-driven primary pump according to apreferred embodiment of the present invention,

FIG. 3 describes a set of bearings used in the preferred embodiment ofthe invention,

FIGS. 4a and 4b describe a flywheel equipped with a vaned wheel used inthe preferred embodiment of the invention,

FIGS. 5a and 5b describe an external cooling circuit used in thepreferred embodiment of the invention,

FIGS. 6a and 6b illustrate an anti-backspin device used in the preferredembodiment of the invention.

For the sake of clarity, the same elements will bear the same referencesin the various figures.

FIGS. 1a and 1b illustrate a motor-driven primary pump according to theinvention and its integration in a vertical configuration into an SMR.FIG. 1a depicts the upper part of a reactor 11 and a motor-drivenprimary pump 10 according to the invention. The upper part of thereactor 11 comprises a pressurizer in which the coolant is kept at highpressure and high temperature under the effect of the controlled nuclearreaction in a lower part of the reactor 11. The reactor primary circuitcomprises one or more motor-driven pumps which circulate fluid betweenthe pressurizer and one or more steam generator(s). The upper part ofthe reactor 11 depicted in FIGS. 1a and 1b comprises a dome-shaped partand a substantially cylindrical base. FIG. 1a depicts the reactor 11equipped with just one motor-driven primary pump 10 in order to make thediagram easier to comprehend. However, it is envisioned that there willbe several motor-driven primary pumps fixed to the substantiallycylindrical base of the upper part of the reactor 11, and around thisupper part. In FIG. 1a , eight motor-driven primary pumps may bearranged around the base, the connection interfaces of the eight pumpsare visible in the diagram. In this example, the design of the reactordictates that the motor-driven primary pump be mounted vertically, headdown. The motor-driven primary pump 10 comprises a motor unit 12 and ahydraulic part 13 which are separated by a thermal barrier 14. The basecomprises part of the primary circuit. As depicted in FIG. 1b , theprimary pumps are connected to a common manifold 15 connected to thereactor core. The coolant is pumped to a common discharge enclosure 16connected to the steam generators. Note too that the fact that themotor-driven pump is integrated into a very tightly confinedenvironment, notably very close to the walls of the reactor places agreat deal of constraints on the design of the motor-driven pump,notably in terms of the flow of the fluid upstream of the impeller,through the impeller and downstream thereof.

The motor-driven primary pump and its integration into the reactor,which are illustrated in FIGS. 1a and 1b , are given by way of exampleand do not limit the present invention. In this example, themotor-driven pump is in a vertical configuration, the coolant isordinary water. More generally, the invention relates to a motor-drivencentrifugal pump for circulating a coolant in a primary circuit of anuclear reactor.

FIGS. 2a, 2b and 2c depict the motor-driven primary pump according to apreferred embodiment of the present invention. The motor-driven primarypump is a motor-driven centrifugal pump comprising a sealed motor unit12, a hydraulic part 13 which is separated by a thermal barrier 14.These three elements have passing through them a shaft 17 rotating abouta longitudinal axis. The rotationally mobile assembly is immersed in thecoolant of the primary circuit.

The motor unit 12 comprises a dry stator 18 and an immersed rotor 19mounted securely on the shaft 17, which turns the shaft 17 and thereforethe mobile elements of the hydraulic part 13.

The hydraulic part 13 comprises an intake duct 20, connected to thecommon manifold 15, and a discharge duct 21 connected to the commondischarge enclosure 16, these ducts being separated by an impeller 22secured to the shaft. Rotation of the impeller 22, driven by the shaft17, allows the coolant to be pumped.

The thermal barrier 14 comprises means of fixing the motor-driven pumpto the reactor. These fixing means are preferably of the stud-nut typeso as to allow ease of disassembly of the motor-driven pump. The thermalbarrier 14 separates the motor-driven pump between a cold partcomprising the motor unit 12 and a hot part comprising the hydraulicpart 13 connected to the primary circuit. Typically, a temperature ofbetween 70° C. and 160° C. is sought for the cold part, while the hotpart is close to the temperature of the coolant, i.e. between 300 and350° C. To achieve that, the motor-driven primary pump according to theinvention comprises a cooling circuit described in detail hereinafter.

The cooling of the reactor is a deciding element in the safety of thereactor. In the event of failure of the motor-driven pump or of theelectrical network powering same, it is necessary to maintain asignificant fluid flow rate in order to maintain a flow of coolant andtherefore maintain a minimum amount of reactor cooling. By way ofexample, this requirement can be expressed quantitatively in terms ofthe requirement to maintain a flow rate greater than or equal to 50% ofthe nominal flow rate 3 seconds after the motor stops as a result of abreak in the electrical power supply. It so happens that the inertia ofthe shaft and of the rotor are not sufficient to meet this type ofrequirement. For that reason, the motor-driven pump also comprises animmersed flywheel 23 mounted securely on the shaft 17 between the hotpart and the cold part of the motor-driven pump or, in other words,between the rotor 19 of the motor unit 12 and the impeller 22 of thehydraulic part 13. The flywheel makes it possible to maintain asufficient coolant flow rate for several seconds after a stoppage orfailure of the electrical power supply. Advantageously, the flywheelalso makes it possible to smooth the rotational speed of the shaft 17and limit jerks when the motor-driven pump is started or stopped. Theflywheel also constitutes a radiological barrier making it possible tolimit radioactive emissions that may escape from the reactor through themotor-driven pump connection interface. The specification of theflywheel has a direct impact on the inertia of the shaft, the frictionlosses and therefore the power of the motor and the capacity of thecooling circuit. It is advantageous to have a flywheel in the form of athick disk, mounted securely on the shaft and made of a material of highdensity, greater than that of stainless steel, and preferably greaterthan 8 kg/l. Advantageously, the flywheel is made of an alloy based onnickel and chromium, preferably of the NiCr₂₂Mo₉Nb alloy known by itsregistered trade name of Inconel 625.

FIG. 3 describes a set of bearings used in the preferred embodiment ofthe invention to ensure reliable rotor dynamics. The shaft 17 and thecomponents connected to it, particularly the rotor 19, the flywheel 23and the impeller 22, forms an assembly 25 with rotational mobility withrespect to a motor-driven pump body 26 made up of the fixed elements ofthe motor unit, of the hydraulic part and of the thermal barrier 14. Themobile assembly 25 is immersed in the coolant circulating through themotor-driven pump. The motor-driven pump comprises a set of bearingswhich are lubricated by the coolant being pumped, ensuring that themobile assembly 25 can rotate with respect to the motor-driven pump body26. In the preferred embodiment depicted in FIG. 3, the set of bearingscomprises:

a first radial bearing 30 positioned between the rotor 19 and an upperend of the shaft,

a second radial bearing 31 positioned between the rotor and theflywheel,

a third radial bearing 32 positioned near the flywheel and on theopposite side of the flywheel to the second bearing,

a fourth radial bearing 33 positioned near the impeller 22.

The first and second radial bearings 30 and 31 guide the rotation of theshaft 17 in a fixed part of the motor unit 12. They guide the rotationof the immersed rotor 19 with respect to the dry stator 18. Thesebearings 30 and 31 are situated in the cold part of the motor-drivenpump.

Advantageously, the bearings 30 and 31 are plain bearings or pad-typebearings and preferably pad-type bearings comprising five pads made ofsilicon carbide with the registered trade name Ekasic, and a stainlesssteel bushing with a tungsten carbide surface treatment.

The third and fourth radial bearings 32 and 33 guide the rotation of theshaft 17 in a fixed part of the thermal barrier 14. These bearings 32and 33 are situated in the hot part of the motor-driven pump.Advantageously, the bearings 32 and 33 are of the hydrostatic and/orthree-lobed bearing type. They may be made from an alloy of the stelliteor Colmonoy® type or may have a surface hardening, obtained by means ofa surface treatment or by means of a coating.

The set of bearings of the motor-driven pump also comprises an axialbearing 34 positioned near the first radial bearing 30 to block theaxial movement of the shaft. In order to allow ease of disassembly ofthe motor-driven pump via the upper part thereof that is freelyaccessible, it is advantageous for the axial bearing 34 to be positionedbetween the rotor and the upper end of the shaft. Advantageously, theaxial bearing 34 is made up of a tilting-pad bearing preferablycomprising fifteen pads made of silicon carbide with the registeredtrade name Ekasic G, and a disk made of silicon carbide with theregistered trade name Ekasic C.

FIGS. 4a and 4b describe a flywheel equipped with a vaned wheel used inthe preferred embodiment of the invention. The rotationally mobileassembly is immersed in the coolant. It is therefore naturally raised toa high temperature. The relatively high rotational speeds of the shaft(typically of between 2000 and 4000 revolutions per minute) generateheat in the bearings and more generally at the rotating parts such asthe flywheel 23 and the immersed rotor 19. For that reason it isnecessary to cool the motor-driven pump and notably the motor unit 12.According to one particularly advantageous feature of the presentinvention, the flywheel 23 comprises a vaned wheel 40 which causes acirculation of the coolant that allows the motor unit to be cooled.

The idea is to use the rotation of the shaft to generate movement in thefluid near the moving parts that are to be cooled. There are a number ofembodiments envisioned for this vaned wheel. It may be made up ofchannels or grooves formed on a surface of the flywheel. In thepreferred embodiment of the invention, as depicted in FIGS. 4a and 4b ,the vaned wheel 40 comprises a plurality of straight fins 41 formed onthe upper surface of the flywheel 23; the upper surface of the flywheelbeing the surface facing toward the rotor. It generates a circulation offluid symbolized by the arrows in FIG. 4b . This circulation on the onehand allows fluid to be fed to an external cooling circuit 42 that willbe described hereinafter, and on the other hand allows a stream of fluidto be generated around the flywheel 23, from the cold part toward thehot part of the motor-driven pump. This stream of fluid notably provideslubrication for the third radial bearing 32 positioned under theflywheel. In the preferred embodiment depicted in the figures, the thirdbearing is a hydrostatic and/or three-lobed bearing. In that case, afeed duct 44 is advantageously formed in the thermal barrier 14 so thatthe coolant can lubricate the third radial bearing 32, as a result ofthe circulation of coolant fluid generated by the vaned wheel 40.

Use of this circulation of coolant caused by a vaned wheel formed on theflywheel is particularly advantageous because it at once allows directcooling of certain components, lubrication of bearings and a supply ofcoolant to an external cooling circuit while at the same time limitingfriction losses. That makes it possible effectively to make a separationat the flywheel between a cold part, i.e. a temperature of the order of80° C. at the upper surface of the flywheel, and a hot part, i.e.temperature of the order of 150° C. at the lower surface of theflywheel.

In an alternative configuration, it is envisioned for the vaned wheel tobe formed on the lower surface of the flywheel.

FIGS. 5a and 5b describe an external cooling circuit used in thepreferred embodiment of the invention. The external cooling circuit 42through which the coolant driven by the vaned wheel 40 of the flywheel23. The cooling circuit 42 comprises one or more coils 50 wound aroundthe motor-driven pump body 26. The coolant circulating through the atleast one coil 50 is cooled by means of a secondary coolant circulatingaround the body 26 in a cylindrical shell 51. The hydraulic circuit ofthe secondary coolant, referred to as the secondary circuit, whichallows the secondary coolant to be pressurized and cooled, has not beendepicted. Any conventional circuit is suited to the present invention.

The external cooling circuit 42 at once allows cooling of the coolant byexchange of heat across the wall of the at least one coil 50 and directcooling of the motor-driven pump by the wall of the motor-driven pumpbody 26.

The acceleration of the coolant that is transmitted by the vaned wheel40 allows the at least one coil 50 to be fed via the feed duct 43 formedin the motor-driven pump body 26. The coolant is cooled as it passesthrough the coil and is then reintroduced into the motor unit near theupper end of the rotor. Advantageously, the fluid is reintroduced via afeed duct 52 formed in the motor-driven pump body 26 facing the firstradial bearing 30 and/or the axial bearing 34. The vaned wheel 40collaborating with the external cooling circuit 42 thus generates afluid circuit that allows the sealed unit to be kept inside atemperature window compatible with its operation and its durability.This device is both simple and highly effective; a cold part kept in atemperature window of between 60 and 100° C. is achieved.

FIGS. 6a and 6b describe an immersed anti-backspin device used in thepreferred embodiment of the invention. The reliability of the pump iskey to the safety of the installation. It is necessary to ensure thatthe direction of pumping cannot be reversed upon a deliberate orunintentional stoppage of the pump. This is because when a motor-drivenpump is stopped while other components are still turning, the pressureon the delivery side of the pump is higher than the pressure on theintake side. A reverse flow passes through the motor-driven pump andturns it in the opposite direction. The speed may be high and exceed thelimit that is acceptable, both on the dynamic, hydraulic and mechanicalviewpoint.

For that reason, the motor-driven pump according to the inventioncomprises an anti-backspin device able mechanically to prevent the shaftfrom rotating in a predetermined direction. The anti-backspin device 60,immersed in the coolant, comprises a fixed part 61 and a mobile part 81secured to shaft. The mobile part has not been depicted in FIGS. 6a, 6band 6c . In a preferred embodiment of the present invention, it is theflywheel 23 described earlier. In that case, the fixed part may beformed on a surface of the thermal barrier 14 facing the flywheel 23.This embodiment is depicted in FIG. 5 a.

The mobile part 81 of the anti-backspin device 60 comprises at least onecavity 80 facing the fixed part 61, and at least one mobile pin 62. Theanti-backspin device 60 is configured so that as soon as the shaft 17 isturned at a sufficiently high speed, the at least one mobile pin 62 isheld inside the said cavity 80 by the effect of centrifugal force. Whenthe rotational speed of the mobile part 81 drops below a predeterminedthreshold, the mobile pin 62 drops down under the effect of gravity andpartially leaves the cavity 80.

The fixed part 61 comprises at least one ramp 63 made up of two inclinedplanes 64 and 65, a first plane 64 of shallow gradient and a secondplane 65 of steep gradient. In the embodiment depicted in FIGS. 6a, 6band 6c , the second plane 65 is vertical. It also comprises anindentation 66 into which the pin 62 may become inserted when it dropsunder the effect of gravity, if the rotational speed is sufficientlylow.

Thus, the principle behind the anti-backspin device is as follows: whenthe motor-driven pump is started up, the pin 62 partially inserted intothe cavity 80 is rotationally driven in the permitted direction andtherefore climbs the shallow-gradient plane 64 before dropping back downinto the indentation 66 formed in the steep-gradient plane 65. When therotational speed exceeds the predetermined threshold, centrifugal forcekeeps the pin 62 in position in the cavity 80. As soon as the speeddrops below this threshold, the reverse happens; the pin 62 drops underthe effect of gravity. If necessary, a reversal of the direction ofrotation of the shaft 17 drives the pin 62 against the indentation 66 ofthe plane 65. The steep gradient of the plane 65 (the plane is verticalin the embodiment depicted in the figures) prevents the pin 62 fromclimbing this plane 65 and thus blocks the rotation of the shaft 17 inthe non-permitted direction.

In other words, the anti-backspin device 60 is considered so that, belowthe speed threshold:

the pin 62 is pushed back into the cavity 80 by contact with theshallow-gradient inclined plane 64, when the mobile part 81 is turningin a permitted direction of rotation,

the portion of pin 62 outside the cavity 80 blocks the rotation throughcontact with the steep-gradient inclined plane 65 when the mobile part81 is turning in the opposite direction.

There are a number of embodiments envisioned for this anti-backspindevice. In the preferred embodiment of the invention depicted in FIGS.6a, 6b and 6c , the mobile part 81 is formed in the flywheel 23. One, orpreferably several, cavities 80 are formed in the lower surface of theflywheel. The cavities have an internal shape that is substantiallycylindrical along an axis parallel to the longitudinal axis of the shaft17. The anti-backspin device therefore comprises as many pins 62 asthere are cavities 80. The pins 62 externally are of substantiallycylindrical shape along a main axis, and are designed to slide in thecavity 80 along the longitudinal axis under the effect of gravity.Likewise, an arc-shaped indentation 66 is formed in the vertical plane65 with a shape suited to the cylindrical pin 62.

As depicted in FIGS. 6b and 6c , the pins 62 may comprise an open-endedduct 70, formed at the center of the pin and substantially parallel tothe main axis of the pin 62. The open-ended duct 70 makes it easier forcoolant to flow inside the cavity 80 as the pin 62 moves.

The at least one pin 62 may also comprise a chamfered profile or arounded profile 71 at its two longitudinal ends. The chamfered profileor the rounded profile 71 depicted in side view in FIG. 6b is there tobreak the right angle between the base of the cylinder and a generatrixof the cylinder. This configuration advantageously makes it easier forthe pin to ascend the shallower-gradient plane 64, and makes it possibleto limit the impact between the mobile part and the fixed part 61 belowthe predetermined speed threshold.

As mentioned earlier on, an anti-backspin device comprising a pluralityof cavities 80 and of pins 62 is envisioned. The cavities 80 aretherefore formed at equal distances from the axis of rotation of theshaft 17 and evenly angularly distributed about the circumference of themobile part 81. For example, the device comprises four cavities 80comprising four pins 62 arranged at four cardinal points of the flywheel23.

An anti-backspin device comprising the same number of cavities 80 and oframps 63 is also envisioned. The ramps 63 may be formed on the fixedpart 61 facing the cavities 80 of the mobile part 81 and evenlyangularly distributed about the circumference of the fixed part 61. Inthe example depicted in FIG. 6a , four ramps 64 are formed at fourcardinal points on a surface of the thermal barrier 14 facing theflywheel 23. Configured in this way, rotational blockage is the resultof contact of each of the pins 62 with a ramp 65 with the fixed part.The torque generated on the shaft is advantageously spread across eachof the pins.

In an alternative configuration, the anti-backspin device comprises adifferent number of cavities 80 and of ramps 63. For example, the devicecomprises the four cavities 80 already mentioned and comprises fiveramps formed on the thermal barrier 14. In that case, rotationalblocking is the result of contact of just one pin 62 with just one ramp65. This configuration offers several advantages. Increasing the numberof ramps 63 makes it possible to reduce the angular travel of the shaftbefore its rotation is blocked. Furthermore, by using a different numberof pins 62 from ramps 63, rotational blockage is performed by a singlepin 62. During successive stoppages of the motor-driven pump, adifferent pin may be called upon to block rotation, thus making itpossible to limit the mechanical stress on the pins.

The anti-backspin device according to the invention is immersed, itscomponents are exposed to high mechanical stresses. The device musttherefore be able to offer a substantial opposing torque in theforbidden direction of rotation, typically of the order of 1000 Nm. Inthe acceleration and deceleration phases below the predetermined speedthreshold, typically of the order of 180 revolutions per minute, thecomponents need to be able to withstand the repeated impact of the pinsagainst the ramps. In order to meet these requirements, the pins areadvantageously made of stainless or, alternatively, of an alloy of theInconel type. It is also envisioned for at least a surface portion ofthe ramps, of the pins or of the cavities to be mechanicallystrengthened using a surface hardening process. What is meant by asurface hardening is a surface treatment or the use of a coating. Asurface treatment using a PVD, which stands for Phase Vapor Deposition,process is envisioned.

Let us note in conclusion that, in the embodiment depicted in thefigures, the motor-driven centrifugal pump is mounted vertically on thereactor. In this configuration, the anti-backspin device 60 comprisescavities configured to allow their pin 62 a vertical movement. Areduction in the rotational speed below the predetermined thresholdcauses all the pins of the anti-backspin device to drop. This embodimentis nonlimiting on the invention. It also for example envisions amotor-driven centrifugal pump mounted horizontally; the axis of rotationof the shaft being horizontal. In that case, the anti-backspin devicecomprises cavities configured to allow their pin a radial movement. Thepins are pressed firmly into the bottoms of the cavity by the effect ofcentrifugal force. A reduction in the rotational speed below thepredetermined threshold causes some of the pins of the anti-backspindevice to drop; the pins situated below the axis of the shaft remainingfully in their cavity. Only the pins situated above the axis of rotationcontribute to the mechanical blocking of the rotation. For thisconfiguration, a high number of cavities, pins and ramps will bepreferred.

1. A motor-driven centrifugal pump for circulating a coolant in aprimary circuit of a nuclear reactor; comprising a sealed motor unit, ahydraulic part and a shaft which is immersed in the coolant, turned bythe sealed motor unit and pumping the coolant by means of an impeller ofthe hydraulic part secured to the shaft; the motor unit comprising a drystator and an immersed rotor mounted securely on the shaft, wherein themotor-driven pump further comprises a thermal barrier between the motorunit and the hydraulic part, and through which the shaft passes, so thatthe motor unit can be thermally isolated; the thermal barrier separatingthe motor-driven pump between a cold part comprising the motor unit anda hot part comprising the hydraulic part, and wherein the motor-drivenpump also comprises an immersed flywheel mounted securely on the shaftbetween the rotor of the motor unit and the impeller of the hydraulicpart, making it possible to ensure a minimum slowing-down time when themotor unit ceases to turn the shaft; and wherein the flywheel comprisesa vaned wheel formed on a surface of the flywheel facing toward therotor, generating a circulation of the coolant around the flywheel fromthe cold part toward the hot part of the pump.
 2. The motor-drivencentrifugal pump as claimed in claim 1, comprising an external coolingcircuit connected to the motor unit, through which the coolant driven bythe vaned wheel circulates; the cooled coolant being reintroduced intothe motor unit.
 3. The motor-driven centrifugal pump as claimed in claim2, wherein the external cooling circuit comprises at least one coilwound on a body of the motor-driven pump; the coolant circulatingthrough the at least one coil being cooled by means of a secondarycoolant circulating around the body in a shell.
 4. The motor-drivencentrifugal pump as claimed in claim 1, wherein the thermal barriercomprises a feed duct configured to allow a radial bearing to belubricated by the coolant, by means of the circulation of coolantgenerated by the vaned wheel.
 5. The motor-driven centrifugal pump asclaimed in claim 1, wherein the vaned wheel is made up of channels orgrooves formed on the surface of the flywheel that faces toward therotor.
 6. The motor-driven centrifugal pump as claimed in claim 1,comprising at least three radial bearings guiding the rotation of theshaft; the rotor being positioned between a first radial bearing and asecond radial bearing; the flywheel being positioned between the secondradial bearing and a third radial bearing; the circulation of coolantgenerated by the vaned wheel also allowing the third radial bearing tobe lubricated.
 7. The motor-driven centrifugal pump as claimed in claim5, wherein the first radial bearing and the second radial bearing arehydrodynamic bearings of the plain bearing, pad bearing and/orthree-lobe bearing type, and the third radial bearing is a hydrostaticbearing.
 8. The motor-driven centrifugal pump as claimed in claim 5,comprising an axial bearing positioned near the first radial bearing andable to block the axial movement of the shaft.
 9. The motor-drivencentrifugal pump as claimed in claim 7, wherein the axial bearing is atilting-pad bearing.
 10. The motor-driven centrifugal pump as claimed inclaim 1, wherein the flywheel is made of Inconel
 625. 11. Themotor-driven centrifugal pump as claimed in claim 1, comprising ananti-backspin device able to mechanically prevent the rotation of theshaft in a predetermined direction.